July 5, 1959
BENZOYL HALIDESWITH TOLCENE IN NON-POLAR SOLVENTS [CONTRIBUTION FROM THE
RICHARD B. WETHERILL LABORATORY O F PURDUE
3303
UNIVERSITY]
Kinetics of the Reaction of Benzoyl Halides with Toluene in Non-polar Solvents under the Influence of Aluminum Halides1n2
c.
B Y FREDERICK R. JENSEN,3z4GIANLORENZO MARIN05 AND HERBERT BROWN RECEIVED OCTOBER18, 1958 Kinetic studies have been made of the ieaction of benzoyl chloride with toluene, under the influence of aluminum chloride, in 1,2,4-trichlorobenzene, o-dichlorobenzene, methylene chloride and ethylene chloride as solvents. The rate constants vary with the solvent, increasing with solvent in the order indicated. The reactions are first order in toluene, first order in the benzoyl chloridealuminum chloride complex, and independent of the concentration of excess benzoyl chloride. The calculated rate constants increase linearly with increasing initial concentration of the benzoyl chloride-aluminum chloride complex, presumably a "salt effect" of the dipolar addition complex. For ethylene chloride solutions 0.1 iM in the complex, the rate constantsare(l0-31. rnole-1sec.-')0.0874at 0.Oo, 0.763at 25.0" and 1.59at35.0°,1eadingtoanenthalpyofactivation of 13.35 kcal. mole-1 and an entropy of activation of -28.1 e.u. To permit independent variation of the aluminum halide, a related study was made of the kinetics of reaction of benzoyl bromide with toluene in the presence of aluminum bromide in 1,2,4-trichlorobenzene as solvent. This reaction is also first order in toluene and first order in benzoyl bromide-aluminum bromide, with the second-order rate constant increasing with increasing concentration of the complex. Under similar conditions the rate constant is only 20% greater than for the corresponding reaction involving benzoyl chloride-aluminum chloride. With aluminum bromide present in excess over the concentration of benzoyl bromide, the rate of the reaction is greatly increased, and a t high concentrations the reaction follows third-order kinetics.
The present program of study of the FriedelCrafts acylation reaction was undertaken with two major objectives. We hoped to utilize the reaction to obtain quantitative data on the effects of substituents on the rates of aromatic substitutions, and to compare the effectiveness of various Lewis acids as catalysts in typical Friedel-Crafts reactions. I n the initial stage of this program, we obtained kinetic data on the aluminum chloride-catalyzed reaction of benzoyl chloride with various aromatic hydrocarbons in nitrobenzene s o l ~ t i o n . ~Un,~ fortunately, kinetic complications were encountered in this solvent.8 The use of benzoyl chloride as a solvent resulted in simple second-order kinetics and the system was applied to the determination of the partial rate factors in the benzoylation of representative benzene derivatives9 and to the study of the effect of various Friedel-Crafts catalysts on the rate.1° Unfortunately, the procedure required exhaustive purification of the benzoyl chloride and a relatively tedious method for following the rate, isolating and weighing the product. Moreover, since the reactant benzoyl chloride also served as solvent medium, it prevented examination of other acid chlorides under conditions where the data would be comparable. For these reasons, it ap(1) T h e Catalytic Halides. XXV. Directive Effects in Aromatic Substitution. XXXI. (2) Based in part upon a thesis submitted by F. R. Jensen in partial fulfillment of the requirements for the Ph.D. degree. (3) Department of Chemistry, University of California, Berkeley, Calif. (4) Research assistant, 1953-1954, on project no. AT(ll-1)-170 supported by the Atomic Energv Commission. h'ational Science Foundation Predoctoral Fellow, 19551955. ( 5 ) Post-doctorate research associate, 1957-1959, on project no. AT(ll-1)-170 supported by the Atomic Energy Commission. (6) H. C. Brown and H. L. Young, J. Org. Chena., 22, 719, 724 (1957). (7) H. C. Brown, B. Bolto and F. R. Jensen, {bid., 23, 414, 417 (1958). (8) N. N. Lebedev, J . Gcn. Chem. (U.S.S.R.), 24, 664 (1954), has observed complex kinetics in the alkylation of benzene catalyzed b y aluminum chloride in nitrobenzene. (9) H. C. Brown and F. R. Jensen, THISJOURNAL, 80, 2291, 2296 (1958). (10) F. R. Jensen and H. C. Brown, ibid., 80,3039 (1958).
peared desirable to explore the possibility of utilizing other, inert solvents for the reaction medium. The kinetics of the Friedel-Crafts acylation reaction in nonpolar solvents has been investigated previously by a number of worker^.^^-'^ I n general, these workers agree that the reaction is first order in the benzoyl chloride-aluminum chloride complex and first order in the aromatic hydrocarbon. Using the reactant benzene as solvent a t 3 5 O , it has been reported recently that the calculated rate constant is essentially independent of the initial concentration of the complex.16 However, the earlier studies, using benzene as solvent a t 3Oo,l2J4toluene as s ~ l v e n t or ~ ~carbon . ~ ~ disulfide as solvent,12are all in agreement that the calculated rate constants increase with increasing initial concentrations of the complex. I n the latter studies the rate constant increased from 15 to 30% for a twofold increase in the initial concentration of the complex. It has also been noted that the excess aluminum halide (over the stoichiometric quantity required to form the benzoyl halide complex) possesses a marked catalytic effect on the acylation reaction. On the other hand, excess benzoyl halide has no effect on the reaction rate, and the ketone is formed irreversibly. Practically all of these earlier studies were carried out in excess aromatic hydrocarbon as solvent or in carbon disulfide. The former expediency was undesirable for one of our objectives. Carbon disulfide possesses a number of undesirable properties for kinetic work. Moreover, it appeared possible that we might avoid or minimize the variation in rate constant with initial concentration of the complex by a judicious choice of solvent. Accordingly, we undertook a kinetic examination of the reaction of benzoyl chloride with a representative aromatic, under the influence of aluminum chloride, in 1,2,4-trichIorobenzene, o-dichlorobenzene, 11,12n14
(11) B. D. Steele, J . Ckcm. Soc., 83, 1470 (1903). (12) S. C. J. Olivier, Rcc. War. thim.,87, 205 (1918). (13) L. F. Martin, P. Pizzolato and L. S. McWaters, Jr.. THIS JOURNAL, 67, 2584 (1935). (14) H.Ulich a n d P. V. Fragstein, Bcr., 72, 620 (1939). (15) F. Smeets and J. Verhulst, Bull. SOC. chim. &lg., 63, 439 (1954).
3304
F. R.JENSEN, G . NARIKO AND H.C. BROWN
methylene chloride and ethylene chloride. Since toluene proved to react a t a convenient rate a t room temperature, i t was utilized as the aromatic in these experiments. The study was extended to include the related reaction of aluminum bromide and benzoyl bromide with toluene in 1,P,-l-trichlorobenzene as solvent. Results Benzoyl Chloride-Aluminum Chloride.-The benzoyl chloride-aluminum chloride complex was prepared by dissolving the two components in the carefully purified solvents. The rates were followed by measurement of the residual benzoyl chloride in aliquots of the reaction solutions. In individual experiments the rates exhibited simple second-order kinetics. The second-order rate constants were calculated from the appropriate integrated form of the rate expression rate = ~ P / C & C H I ][CsH~COCI.AIC13]
In the initial exploratory experiments, we were content with a precision of ~ k 5 7 0in the rate constants. These data are summarized in Table I, with the rate constants rounded off to two significant figures. We then adopted ethylene dichloride as the reaction medium of preference and made a more detailed study, with higher precision, +2%, of the effects of varying the concentrations of the reactants. These data are summarized in Table 11. RATE
CONSTANTS
---
:CaHiCOCI.
AlClil
s1
TABLE I1 CUSSTXNTS FOR THE REACTION OF BE RIDE-ALUMISUM CHLORIDE WITH TOLUETE IN ETHYLENE DICHLORIDE
KATE
--
Reactants, .If-------
ICeHKOCI. AlC1a1
Tcrnil., OC.
25.0 25 0 25 0 25.0 25 0 25.0 25.0 25,o 25 0 25 0 25.0 0.0 35 0
(i 100
0
100
n
,100 100 100 .l50 200 200 ,240 300 ,300
0 0 100 0 200 0 0 0 1)
0 0
. 100
0
3 00
0
,
Rate constant fh
[CsH&XXl]
x
10.3.
[CeHsCN31 1. m o l e - ]
bet. - 1
u . 763
0 . 100 200 300 100 . 100 ,150 ,100 , 200 ,240 ,100 ,300 . 100 , 100
--7
, 1 1 1
,771 776
. ?-. I l h 849 !)BO ,965 1 01
1.08 1 ,011 0.0874 1.59
solubility of free aluminum chloride in these solvents made it necessary to utilize the soluble aluminum bromide in studies of the efTect of excess metal halide. Solutions of aluminum bromide in 1,2,4-trichlorobenzene proved to be stable over periods of several weeks. Consequently, this solvent was adopted for the kinetic measurements. Benzoyl bromide was utilized as the acid halide to avoid the complication of halogen exchange between the metal halide and the acyl halide. The experimental results are summarized in Table 111.
FOR THE
RIDE-.~LUXISUM
Solvent
TABLE I REACTION OF BENZOYL CHLOCHLORIDE WITH TOLUENE AT 25.0"
L-01.
Kate constant Reactants .11--~[CaHsCOCI 1
~
!CtHsCHij
(i 0942 TCB" n 0 0942 TCB 0906 0 276 TCB 275 0 275 TCB 2i6 0 303 277 TCB 274 0 0992 TCB 642 (i 277 DCB* 0949 289 0 293 DCB 293 i) DCB 677 (i 289 0973 31CC 1) 289 307 51C 0 307 MC 312 0 204 312 307 652 0 51C a 1,2,4-Trichlorobenzene. o-Dichlorobenzene ylene chloride.
x
104,
I. mole-. sec. - 1
0 18 18 30 28
30 50
4.1 4 3
78 6-3 78 85
TABLE 111 RATE COXSTAXTS F O R T H E REACTTOS O F BESZOYLB K O VIDE-.%LUMISUM BROVIDE WITH TOLUEXE AT 25.0" I N 1,2,4-
TRICHLOROBEKZENE
---
R a t e constant
Reactants. '11 [Ci,HsCOBr.A113rs1 [CbHsCHil
0 312
,311 ,310 ,0242 . 0j26 105 200 309
,311
604
30; ,309 ,301 309 ,312
,6iF
Meth-
.Ti4
The results show that the magnitude of the rate constants increase with the solvent in the order: 1,2,4-trichlorobenzene < o-dichlorobenzene < methylene chloride < ethylene chloride. However, the kinetic features of the acylation reaction 'ippear to be the same in the four solvents. X change in the initial toluene concentration causes no change in the calculated second-order rate constants, whereas an increase in the initial concentration of the complex results in a moderate increase in the value of the rate constant. The addition of excess benzoyl chloride causes no significant change in the magnitude of the constant. Benzoyl Bromide-Aluminum Bromide.-The in-
,311 ,309 ,308
,ion
-1 (
0.156 ,311 ,617 ,311 ,311
1.025 1.024
k2
x
1 m,ie-:
103, ~ C C -1.
i l 420
423 422 . 21111 . .?.,,q --
"is :3,'30
3s; ,52? . ,j-;, r )
,782 . 895 92.5
The reaction of benzoyl bromide-aluminum bromide with toluene exhibits characteristics similar t o those of the chloride reaction. For any initial concentration of the complex, the reaction appears to be clearly second order. Changes in the toluene concentration from 0.156 to 0.6117 1 1 have no significant effect on the rate constant. However an increase in the initial concentration of the complex produces an increase in the calculated second-order rate constant. The increase in the rate constant with increase in concentration of the complex is very similar for both the chloride and bromide (Fig. 1). The two reactions are quite similar in
rate. At the same concentration (0.3 M ) in 1,2,4trichlorobenzene, the benzoyl bromide-aluminum bromide complex reacts with toluene 20% faster then does the corresponding chloride complex. The marked catalytic effect of aluminum halide present in excess over the concentration of the acid halide has been noted OlivierI2 reported that in the presence of excess aluminum bromide the data for individual experiments followed a second-order rate expression and the value of the second order rate constant varied with the concentration of free aluminum bromide. At relatively low aluminum bromide concentrations, we also observed second-order kinetics (Table 111). However, a t somewhat higher aluminum bromide concentration, the reactions appeared to be pseudo third order (Table IV).
-
cen5coci
Time, min.
M
0.154
0.309
0.617
a
=
Reactants" ---Rate constantsk3,C 1.2 [CeHaCOBr. AIBrz] kz,b mole-% 'M 1. mole-' sec.-i sec.-l
0 2.84 8.15 13.11 23.48 28.07
0.309 ,230 ,1578 ,1255 ,0839 ,0752
0 1.90 3.64 5.10 6.76 8.30 9.84
0.309 ,1637 ,1242 ,1093 ,0955 ,0848 .0788
..... 0.0187 ,0159 ,0150 ,0148 ,0138
0 0.84 2.25 3.77 5.89 8.14 9.76
0.309 .l52 .0998 ,0792 ,0652 ,0548 ,0481
0.0407 .0345 ,0290 ,0267 ,0265
[C,H,CH$]o = 0.309 M .
koia
-
..... 0.00625 .00588 ,00603 ,00591 Xv. 0 00603
.... 0.0303 ,0355 ,0493 ,0515
0.337 ,326 ,318 ,328 ( ,365) Av. 0.327 Rate = & [ a - XI*. Rate
4 . 3
The rate constants for the kinetic studies with excess aluminum bromide are summarized in Table V. With initial concentrations of toluene and the addition compound of 0.3 M , the initial rate of reaction is increased about 100-fold by the presence of an additional mole of aluminum bromide (AlBr3). The presence of two molar quantities of aluminum bromide increases the initial rate of reaction by a factor of approximately 500. Discussion Kinetics of the Benzoyl Halide-Aluminum Halide Reaction with Aromatics.-The present study con-
,'
,'
/'
__I,-
.........
,'.'
,_-"
0.4
00
.
d
O,,," ,I
41C$
In T C B
i
0
02
04
06
00
Complex concn., -IT. Fig. 1.-Change in the second-order rate constant with change in the initial concentration of the acid halidealuminum halide complex.
TABLE V RATE COXSTANTS FOR THE REACTIONOF BENZOYL BROMIDE--~~LUMINUM BROMIDE WITH TOLUEXE IS THE PRESENCE OF EXCESS ALUMINUM BRGMIDE IN ~ , ~ , ~ - T R I C H L O R O B E N Z E N E AT 25"
.... 0.132 ,121 .126 ,132 .130 Av. 0.128
,_
AICIWMC
C*H,COCI
TABLE IV RATE DATAILLUSTRATING THE DEPENDENCE OF THE KINETICORDERON THE CONCENTRATION OF EXCESS ALUMINUM BROMIDEIN THE REACTIONOF BENZOYL BROMIDE-.%LUMINUM BROMIDEWITH TOLUENE IN 1,2,4-TRICHLOROBENZENEAT 25' Excess AlBn
3305
BENZOYL HALIDESWITH TOLUENE IN NON-POLAR SOLVENTS
July 5, 1959
-
[AlBn]
Reactants,-4A [CeHsCOBr. AlBrs]
[CsHCHs]
0 0.154 ,309 ,465 .617
0.309 ,300 ,309 .311 .309
0.309 ,300 .309 ,310 .309
-Rate constankz X 103, ka X lo%, 1. mole-1 1.1 mole-* sec. sec. - 1
0.405
6.05 128 225 327
firms earlier reports that the reaction of benzoyl halide-aluminum halide is first order in each component, but that the second-order rate constants increase with increasing initial concentration of the acyl halide-metal halide addition compound."J4 The sole exception to this conclusion is provided by the recent data of Smeets and VerhulstL5for the reaction in benzene a t 35'. These authors followed the reaction by sweeping out the evolved hydrogen chloride and titrating it, whereas we and Olivier12 followed the reaction by determination of residual benzoyl halide. In spite of different analytical procedures for the acid halide, our kinetic results and conclusions are in excellent agreement with those of Olivier where they can be compared. Unfortunately, Smeets and Verhulst failed to discuss this discrepancy between their results and those of earlier workers.lB Until this discrepancy has been resolved, we shall proceed on the basis (16) T h e Referee has pointed out that the kinetics may be influenced by the partial pressure of hydrogen chloride and this may be responsible for the difference in results.
3306
F. R.
JENSEK,
G. ~ T A R I X O
indicated by the majority of the data that the rate constant for the reaction of the benzoyl halidealuminum halide addition compound with simple aromatics in non-polar solvents increases generally with increasing initial concentration of the complex. There is considerable evidence that the rate of the benzoylation reaction is quite sensitive to the polarity of the reaction medium. Thus the reaction rate increases moderately as the solvent is altered from 1,2,4-trichlorobenzene, to o-dichlorobenzene, to methylene chloride, to ethylene chloride, with a further large increase with benzoyl chloride (Table VI).
AND
H. C. BROWN
DERIVEDDATAFOR
lro1. 81
TABLE VI1 BENZOYLATIOS OF BESZEXEA N D TOLUENE
THE
Energy of activation
Enth:lpy
Eact,
Compound
Solvent
kcal./ mole
Benzene BC" 15.8 Toluene BC" 13.3 Toluene ECb 13.9 Benzoyl chloride (ref. 9).
log sec.
+;
activation AH+, Entropy of kcal./ activation mole AS+, e.u.
15.1 -27.2 12.6 -96.7 13.3 -28.1 7.08 Ethylene chloride. 7.28
7.49
two mechanisms which appear possible for the benzoylation reaction,g,lo a direct reaction of the aromatic with the addition complex or a prior TABLE VI ionization of the complex followed by reaction of COMPARISON OF RATESOF REACTION OF BENZOYL CHLO- the aromatic with the ion-pair intermediates.1ga20 RIDE-A4LUMINUMCHLORIDE WITH TOLUENE AT 25.0" The present data do not distinguish between these Rate constant," mechanisms.21 A complete understanding of the flz x 103. Relative Solvent I. mole-' 5ec rate aluminum chloride-catalyzed acylation mechanism 1,2,4-Trichlorobenzene 0.18 1.0 requires a better understanding of the structures o-Dichlorobenzene .43 2.4 of the acyl chloride-aluminum chloride comMethylene chloride .63 3 5 p l e ~ e sas~ well ~ , ~as ~ information as to the extent Ethylene chloride 0 76 4 2 and rate of ionization of these complexes in various 5 43 30 Benzoyl chloride reaction media.24 a Concentration of complex, 0.10 M. The reaction of benzoyl bromide-aluminum bromide with toluene appears to be essentially On this basis the increase in rate with increase in identical with the related reaction of the chlorine the initial concentration of the benzoyl chloride- derivative. The kinetics are the same, the rate aluminum chloride, observed in the non-polar reac- constants exhibit the same change with initial comtion media, might be the result of a solvent eFect. plex concentration, and the actual second-order rate These addition compounds are highly polar sub- constants differ by only a small factor (20%). stances with high dipole moment^.'^ Their pres- Consequently, it is probable that both reactions ence in the non-polar solvents would be expected proceed through identical paths. to modify significantly the polarity of the medium. In the presence of excess methyl bromide, diThe reaction products, R2C0.A41C13, have similar meric aluminum bromide, AlzBrs, dissociates to dipolar properties. Olivier12demonstrated that the form the 1:l compound, CH3Br:A1Br3. In the addition of benzophenone-aluminum bromide to a presence of excess aluminum bromide, the 1:2 typical reaction mixture resulted in the same in- compound forms CHsBr :A12Br6.25 Presumably crease in the observed rate constant as an equiva- the addition of excess aluminum bromide to the lent increase in the initial concentration of benzoyl 1: 1 beniopl bromide-aluminum bromide complex bromide-aluminum bromide. Consequently, the results in its conversion to a 1 : 2 derivative, Cgpolarity of the solvent should depend primarily HsCOBr.Ak12nr6.12 The marked increase in reaction on the initial concentration of the complex and rate accompanying the introduction of excess would not be expected to change significantly as aluminum bromide is readily accounted for in the reaction proceeds. terms of the greater electrophilicity of the 1: 2 comBenzoyl chloride itself is a highly polar reaction plex, or in terms of its more extensive ionization. medium. I n this solvent the rate of the benzoylaThe alteration in order from second, with minor tion reaction is independent of the initial concen- amounts of excess aluminum bromide, to third tration of the c ~ m p l e x . ~ Consequently, the ex- with larger amounts, is more difficult to explain. perimental observations are consistent with a "salt Although i t is possible to devise kinetic schemes effect" of the addition compounds in these non- which account for these changes in reaction order, polar solvents. However, it should be pointed out these schemes must be considered highly speculathat alternate interpretations, such as a weak cat- tive in the absence of direct knowledge of the moalytic effect of the addition compounds, are also lecular species present in the solutions. Consepossible. (19) G. Baddeley and D. Voss, J . Chem. Soc., 418 (1954). In spite of the difference in kinetics for the ben(20) For a detailed review of these two mechanisms see P. H. Gore, zoylation reaction in benzoyl chloride and in these Chemistry b Indicstry, 1388 (1954); Cltem. Rets., 65, 229 (1955). (21) Recently the preparation of several ionic acylonium fluor0non-polar solvents, the actual substitution stage RCO+BFd-, was described and their ready reaction with must be quite similar. This is indicated by the borates, aromatics noted; G. Olah and S. Kuhn, B e y . , 89, 8fA (19.50). similarity in the entropies of activation for the two (22) S . X. Lebedev, J . Gew. Chen7. (L'.S.S R,), 21, 1788 (1951). reactions (Table 1-11), the similarity in the (23) B. P. Susz and I. Cooke, Helv. C,him. A c t o , 37, 1?79 (1954); toluene/benzene reactivity ratiola and the simi- 1. Cooke, B. P. Susz and C. Herschmann, i j l i d . , 37, 1280 (1951); B. P. Susz and J. J. Wuhrmann, ibid., 40, 971 (1957). larity in the isomer distribution.18 (24) J. L. Huston and C. E. Lang, J . Inorg. Nucl. Chrm., 4 , 30 I n previous publications we have considered the (1957). -1
(17) W. Nespital, 2. physik. Chem., B16, 153 (1932). (18) H. C.Brown and G. Marino, TEIS JOURNAL, 81, 3308 (1959).
(25) H . C. Brown and W. J. Wallace, THIS JOURNAL, 76, 6279 (1953).
Jury 5, 1959
3307
BENZOYL HALIDES WITH TOLUENE IN NON-POLAR SOLVENTS
quently, discussion of this phase of the investigation will be deferred. In it appears that the Of benzoyl chloride-aluminum chloride with toluene in 1,2,4-trichlorobenzene or ethylene chloride is relatively simple kinetically and closely related to the previousb' studied reactions of this complex in benzoyl chloride ~ o l u t i o n . The ~ simplicity of the experimental technique should facilitate the determination of the effectsof substituents in the aromatic on the substitution rates. The variation in the rate constant with initial concentration represents a minor difficultv which should be cir'urnvented either by 'Omparing rate 'Onstants a t infinite dilution or by comparing them a t the same concentrations. The related simple kinetics observed for the corresponding reaction of benzoyl bromide-aluminum bromide with toluene in 11214-trich10robenzene supports the that this system should also prove applicable to a study of the effect of various metal halides on the rate,as well as the investigation of the influence of the structure of the acyl halide upon the reaction.26
Experimental Part Materials.-The 1,2,4-trichlorobenzene was isolated from a mixture of isomers (Hooker Electrochemical Co.) by washing with fuming sulfuric acid, followed by water, drying over calcium sulfate, and then slowly fractionating the material through a column rated a t 70 plates. o-Dichlorobenzene, methyl chloride and ethylene chloride were commercial products which were treated with fuming sulfuric acid, washed with water, and distilled in the same column. I n all cases, only the central fractions of constant boiling points were used. Benzoyl chloride was purified in the manner described previously.0 Benzoyl bromide was twice distilled through the 70-plate column, packed with glass helices, under reduced pressure, with careful protection from air and moisture. The preparation of pure colorless aluminum chlorideg and aluminum bromide27 has been described in previous publications in this series. Preparation of Solutions.-Standard solutions of the complex, CaHbCOCl.AIC13, in the various solvents were prepared by adding equivalent amounts of benzoyl chloride and aluminum chloride to the appropriate solvent, with careful exclusion of moisture. -4glass enclosed magnetic bar was added and the solution was stirred vigorously to facilitate the formation of the complex. Since the aluminum chloride dissolved relatively slowly, the solutions were usually warmed to facilitate the process. Although the solutions were originally colorless, warming frequently resulted in a slight yellowing. h'o further color change was visible following the addition of toluene. These slight colors appeared to be due to the presence of trace quantities of impurities. Since the rate constants proved to be readily reproducible and did not appear to be influenced by the presence or absence of these slight colors, we concluded that they had no significant effect upon the kinetics. To test possible reaction with the solvent, a standard solution of benzoyl chloride-aluminum chloride in 1,2,4(26) A study of the reaction rates exhibited by a number of substituted benzoyl chlorides has been completed by G . Marino and will be reported shortly. (27) €1. C . Brown and H. Jungk, THIS JOURNAL, 77, 5579 (1955).
trichlorobenzene was prepared. Analysis for benzoyl chloride indicated the presence of 98.3Yc of the calculated amount. After 48 hours at 25', the solution was again analyzed. The concentration of benzoyl chloride was unchanged. A solution of aluminum bromide in 1,2,4-trichlorobenzene was prepared and allowed to stand for several weeks. The solution remained clear, with no noticeable change. After hydrolysis, the organic material exhibited properties identical with those of the original solvent. I n reactions involving aluminum bromide, advantage was taken of its solubility by preparing standard solutions of both aluminum bromide and of benzoyl bromide in 1,2,4trichlorobenzene and preparing the reaction so~utions by mixing measured volumes of these two solutions with the solve$. The special apparatus and techniques developed previously for handling benzoyl chloride solutions9 were utilized. Kinetic Measurements.-The reaction solutions were measured in the special storage flasksQand introduced into the reaction vessels. The latter were connected to a dry nitrogen supply and a small pressure was maintained in the system so that removal of the stopper automatically resulted in a flow of nitrogen through the narrow neck of the flask. After the solutions had reached temperature equilibrium with the bath (&0.03'), the reaction was initiated by adding the toluene. Aliauots for analvsis were withdrawn in the steam of nitrogen using fast delivery pipets which had been calibrated for delivery with the individual solvents. The aliquots were analyzed volumetrically for residual benzoyl chloride iising the procedure previously developed for studying the rate in nitrobenzene s ~ l u t i o n . ~ For the preliminary survey in the various solvents, we were content with a precision of & 5 % (Table I ) . For the remaining study (Tables I1 and 111) we sought a higher precision, &2Yc. A typical study is summarized in Table VIII. TABLE VI11 TYPICAL DATAFOR THE REACTION O F BENZOYL CHLORIDEALUMINUM CHLORIDE w I m TOLUENE IN ETHYLENE CHLORIDE SOLUTION AT 25.0 Time, min.
NaOH, ml.
Reactio~,~
7%
2 a
-
x
Rate constantb
kz
x
103,
1. mole-1 set.-'
0 8.82" 0.79 2.00 8.75 6.60 25.17 71.0 0.3363 0.789 170 4.90 ,784 44.44 0.7998 248 53.23 4.12 1.1404 .766 285 3.75 57.48 1.3515 .790 362 3.26 66.04 1.7056 .785 462 2.85 67.69 2.0950 .772 532 2.55 71.09 2.4590 . 770 Initial concentrations: [CBHjCH3]0.100 M ; [CeHsCOCl~AlCla] 0.100 M. * Graphical evaluation: kf 0.779 X 1. mole-' set.-'. Calcd. (I
The precision attained is also indicated by the agreement between the rate constants in individual determinations: [C~H~COC~.A~C [CsH,CH3] II], 0.100 M ,k2 = 0.778, 0.741, 0.779, 0.755: ICeHaCOCl.AlClr1 IChH5CHI1 0.200 A[, ki = 0.952, 0.999, 0.947, 0.964;-' [ ~ ~ H , C O C ~ : A ~ [CQH5; C~I], CHI] 0.300 M , kz = 1.075, 1.100, 1.089. Average values for the rate constants are listed in Table 11. LAFAYETTE, IND.
